Frost and ice accumulation are a major safety and efficiency concern for aircraft, sea vessels, wind turbines, off-shore oil platforms, and other critical infrastructure. A common and effective way of preventing ice accretion is to dispense freezing point depressants. For example, prior to take-off in wintery conditions, airplanes are often sprayed with antifreeze liquids such as ethylene glycol. To prevent antifreeze depletion during flight, commercially available weeping wing systems can continuously dispense antifreeze over the wings through an active pumping system (see for example Ryerson in C. C., in “Assessment of superstructure ice protection as applied to offshore oil operations safety: ice protection technologies, safety enhancements, and development needs”, in Engineer research and development center, Hanover, N.H. cold regions research and engineering lab: 2009). For small airplanes, about 4 to 8 liters of the antifreeze liquid are dispensed per hour. However, the high cost of the active dispensing system and fairly large quantities of antifreeze make it unlikely to be adopted in other applications such as large airplanes and ships. Moreover, the complexity of these systems can limit their scalability for emerging miniaturized devices such as unmanned aerial vehicles.
In order to reduce the use of antifreeze, and eliminate complex dispensing systems, semi-porous coatings that wick in the antifreeze from a reservoir have been developed (see for example the Feltwick Grating in Ryerson, and Chang, Y. S., in Performance Analysis of Frostless Heat Exchanger by Spreading Antifreeze Solution on Heat Exchanger Surface. Journal of Thermal Science and Technology 2011, 6, (1), 123-131, and Chang, Y. S.; Yun, W. N., in An Experimental Study on the Frost Prevention using Micro Liquid Film of an Antifreeze Solution. International Journal of Air-Conditioning and Refrigeration 2006, 14, (2), 66-75). However, in these systems, the antifreeze can be rapidly diluted by rain or atmospheric precipitation, or by sea water spray created by spindrift spray ripped from tops of waves by wind, or by splashing of sea water against the ship (see Ryerson cited above).
As an alternative to active anti-icing systems, a number of passive bio-inspired coatings have been recently proposed; however, their functionality is often limited to a particular set of environmental conditions. Examples of such coatings include nanostructured superhydrophobic surfaces that slow ice accumulation by repelling sub-cooled water droplets, but when frosted over, increase ice accretion and adhesion (see for example Varanasi, K. K.; Deng, T.; Smith, J. D.; Hsu, M.; Bhate, N., in “Frost formation and ice adhesion on superhydrophobic surfaces”, Applied Physics Letters 2010, 97, (23), 234102). Similarly, it was recently demonstrated that the condensation and frost inhibiting functionality of lubricant-impregnated surfaces (LIS) can be compromised due to lubricant drainage into nucleating frost consisting of network on nano-iciles (see for example Rykaczewski, K.; Anand, S.; Subramanyam, S. B.; Varanasi, K. K., in “Mechanism of Frost Formation on Lubricant Impregnated Surfaces”, Langmuir 2013, 29, (17), 5230-5238).
The onset of frost growth can occur directly through desublimation or indirectly through condensation of droplets followed by their freezing (i.e. condensation frosting). In both desublimation and condensation, the nuclei formation rate per unit area, I, can be expressed according to Becker-Döring embryo formation kinetics as:I=I0exp(−ΔGc/kTsur)  (1)
where I0 is the kinetic prefactor, ΔGc is the critical Gibbs energy change for nucleation, Tsur is temperature of the surface and k is the Boltzmann constant. At the critical embryo size, ΔGc is defined by:
                              Δ          ⁢                                          ⁢                      G            c                          =                              4            ⁢            π            ⁢                                                  ⁢                          σ              3                        ⁢                                          V                m                2                            ⁡                              (                                  2                  -                                      3                    ⁢                    cos                    ⁢                                                                                  ⁢                    θ                                    +                                                            cos                      3                                        ⁢                    θ                                                  )                                                          3            ⁢                                          (                                                      R                    _                                    ⁢                                      T                    sur                                    ⁢                                      ln                    ⁡                                          (                                              P                                                  P                          sat                                                                    )                                                                      )                            2                                                          (        2        )            
Where θ is the contact angle of the embryo in contact with the solid, σ is either the liquid-vapor or the ice-vapor surface energy, R is the ideal gas constant, and Vm is the molar volume of the liquid, P is the partial pressure of water vapor in the surrounding, and PSAT is the water vapor saturation pressure at the surface temperature. Equations 1 and 2 imply that frost formation can be significantly delayed (i.e. nucleation rate decreased) by increasing θ. Since it only requires modification of the surface chemistry, this approach to frost inhibition has been thoroughly studied.
To surpass the limits of hydrophobicity created by chemistry alone, nanoscale and microscale surface texturing has been used to develop superhydrophobic anti-frosting surfaces. However, the macroscopically high contact angle of such surfaces (>150°) does not necessarily translate into a decrease of the nucleation rate compared to flat hydrophobic surface because nuclei can form in-between the surface features or follow alternative growth pathways. On some superhydrophobic surfaces, however, frost growth can be delayed through other physical mechanisms such as condensate ejection upon coalescence. Another approach for spatial control of vapor nucleation is disruption of a hydrophobic surface with hydrophilic patterns onto which droplets can condense preferentially. Other work has demonstrated that the spread of frost can be prevented by intelligently spacing these preferential condensation areas. Specifically, if the droplets are spaced sufficiently far apart, they cannot be connected by an ice bridge originating from one of them (the ice bridge acts as a humidity sink and its growth is fed by evaporation of the unfrozen droplet). While promising, it is unclear if this effect, as well as droplet ejection, can be used to inhibit frost in industrial setting where hydrophobic coatings are prone to damage.
In addition to decreasing surface wettability, Equations 1 and 2 suggest that the onset of frost formation can be significantly delayed through the decrease of the nucleation rate via reduction of the local water vapor pressure above the surface. This task is significantly more difficult to achieve than engineering reduced surface wettability and has only recently been explored. Some studies have demonstrated that water vapor pressure can be locally depressed below the saturation pressure through the presence of a hygroscopic material. Specifically, a region of inhibited condensation and condensation frosting (RIC) forms around a salty water drop. The water vapor concentration at the surface of a hygroscopic liquid (from ideal gas law CHS=PHS/RTHS=PHS/RTsur) is lower than the saturation concentration (CSAT=PSAT/RTsur), producing a locally decreased water vapor concentration that can be viewed as a “humidity sink”. The RIC has also been observed during condensation of water and of diethlyene glycol as well as during nucleation of calcite crystals. Other studies have shown that a significant delay in surface frosting can be achieved by arranging drops of hygroscopic liquid in an orderly array so that the individual RICs overlap (e.g., see Sun, X.; Damle, V. G.; Uppal, A.; Linder, R.; Chandrashekar, S.; Mohan, A. R.; Rykaczewski, K. Inhibition of Condensation Frosting by Arrays of Hygroscopic Antifreeze Drops. Langmuir 2015, 31, 13743-1007 13752. This integral humidity sink effect was also implied by the long delay in condensation observed on bi-layer, antifreeze-infused, anti-icing coatings. These coatings consist of a porous, superhydrophobic, outer layer and a hydrophilic inner layer that contains a hygroscopic antifreeze (all common antifreezes are hygroscopic). This hybrid coating delays the formation of glaze and rime by repelling impacting drops and, once the pores are flooded, counteracts frost formation through the diffusive release of the antifreeze (presence of this liquid also lowers ice adhesion once if it eventually forms). This approach is a hybrid between fully passive coatings and antifreeze flooding technique commonly used in industry. Instead of getting rid of the antifreeze entirely, this anti-icing method reduces its operational costs and environmental impact by minimizing the amount of antifreeze used (up to 8-fold). The observed delay in frost formation as well as pinning of impacting drops during freezing rain suggests that the water vapor pressure was altered above the bi-coating.
Accordingly, there exists a need to provide surfaces or coating technologies that provide anti-icing functionality for structures and vehicles of all sizes operating in a wide range of environmental conditions on land, on the sea, and in the air.